The ever-growing demands on the functionality of automatic transmissions arising from the need for greater spontaneity, the ever-increasing number of gears to be shifted, the consumption-optimized design of automatic transmissions with larger proportion of driving in the high gears and the large number of downshifts to be carried out when braking the vehicle to a standstill, entail that shifting between the gears of an automatic transmission has to be done ever more rapidly and frequently. Particularly for a sporty style of driving, the spontaneity of shifts is an important factor.
To carry out a gearshift, the shifting element to be shifted first has to be prepared to transmit torque, i.e. it must be filled, and rapid filling results in an increase of the shifting speed.
Multiple-disc clutches of all-wheel transfer transmissions, referred to as all-wheel disc clutches in what follows, have the special feature that as a rule they are permanently operated with slip; when an all-wheel disc clutch is actuated, a torque that is adjustable as a function of the actuation is transmitted to all the drive axles of the vehicle.
Since all-wheel disc clutches are operated permanently with slip, they are designed to be robust; furthermore, they have to meet particular requirements relating to adjustment accuracy, adjustment dynamics and the dynamics of the opening process, and to achieve good adjustment dynamics, an all-wheel disc clutch has to be filled as rapidly as possible.
From the prior art it is known to fill disc shifting elements in a time-controlled manner, the filling being carried out with reference to the motor speed at the moment when the filling process begins.
For example, from DE 100 36 510 C2 by the present applicant it is known to determine the filling level of a shifting element in an automatic transmission, both during filling and during draining, by means of an algorithm stored in the electronic control unit and, by means of the algorithm, to carry out the respective shifting element control for filling and the control of the shifting element for torque uptake and torque transmission. In this case, the duration of the shifting element's filling phase is determined by virtue of the algorithm.